Cellular functions in infection by DNA tumor virus (papovaviruses and adenoviruses) are being investigated with particular reference to DNA synthesis and their role in permissivity to virus replication. Emphasis is placed on obtaining cell mutants in mouse (3T3) and Chinese hamster (CHO) cells, including those temperature-sensitive in DNA, utilizing the techniques of replica plating and recombinant DNA among others. Mutants will be analysed for their efficiency in synthesis of viral DNA in vitro. SV40 infection of human diploid fibroblasts is being studied in relation to viral DNA synthesis in an effort to determine the basis for the semipremissive interaction and its effect on the efficiency of viral transformation.